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<h1><a href="https://archiveofourown.org/works/29014719">I Who Have Loved the Stars</a> by <a class='authorlink' href='https://archiveofourown.org/users/LadyHallen/pseuds/LadyHallen'>LadyHallen</a></h1>

<table class="full">

<tr><td><b>Category:</b></td><td>Harry Potter - J. K. Rowling</td></tr>

<tr><td><b>Genre:</b></td><td>Alternate Universe - Canon Divergence, Angst, Fluff, Gen, Medical Inaccuracies, Reincarnation, Self-Insert, Twin AU, outsider pov</td></tr>

<tr><td><b>Language:</b></td><td>English</td></tr>

<tr><td><b>Status:</b></td><td>Completed</td></tr>

<tr><td><b>Published:</b></td><td>2021-01-27</td></tr>

<tr><td><b>Updated:</b></td><td>2021-01-27</td></tr>

<tr><td><b>Packaged:</b></td><td>2021-05-18 10:27:29</td></tr>

<tr><td><b>Rating:</b></td><td>Mature</td></tr>

<tr><td><b>Warnings:</b></td><td>No Archive Warnings Apply</td></tr>

<tr><td><b>Chapters:</b></td><td>1</td></tr>

<tr><td><b>Words:</b></td><td>3,430</td></tr>

<tr><td><b>Publisher:</b></td><td>archiveofourown.org</td></tr>

<tr><td><b>Story URL:</b></td><td>https://archiveofourown.org/works/29014719</td></tr>

<tr><td><b>Author URL:</b></td><td>https://archiveofourown.org/users/LadyHallen/pseuds/LadyHallen</td></tr>

<tr><td><b>Summary:</b></td><td><div class="userstuff">
              <p>No one had expected Cassiopiea to be born.</p><p> </p><p>Or a Self-Insert is reincarnated as Regulus Black's Twin and does her best.</p>
            </div></td></tr>

<tr><td><b>Relationships:</b></td><td>Regulus Black &amp; Original Female Character, Regulus Black &amp; Sirius Black</td></tr>

<tr><td><b>Comments:</b></td><td>42</td></tr>

<tr><td><b>Kudos:</b></td><td>696</td></tr>

</table>

<a name="section0001"><h2>I Who Have Loved the Stars</h2></a>
<div class="story"><div class="fff_chapter_notes fff_head_notes"><b>Author's Note:</b><blockquote class="userstuff">
      <p>I have been wanting to write this for ages, so I hope I did it justice.</p>
    </blockquote></div><div class="userstuff module">
    
    <p>Sirius blamed the war for the high shriek he let out.</p><p>When tensions are high and you were at great risk for being murdered for what you believed in, knocks at what you’re completely sure is a secure apartment is not a good thing to hear in the middle of the night.</p><p>“Sirius, open the fuck up or I’ll tell James who ate his special brownies!” a voice yelled beyond the door.</p><p>He blinked at it for a second, and then he realized, “Cassie, you asshole, why are you knocking on people’s doors at 2 in the morning?”</p><p>Still, he opened it. She could be bleeding to death. With Cassie, you never know.</p><p>He almost shrieked again. Cassie, bleeding from the leg and leaning heavily on the door jamb. Regulus, leaning on her shoulder and bleeding just as much from his shoulder with the complexion of a corpse.</p><p>“Merlin’s fucking balls,” he swore. “Did you just bring Reggie’s dead body here?”</p><p>Cassie shouldered him out of the way. “No, idiot.  Do you have any potions here?”</p><p>Sirius chased after her, taking Regulus’s other arm and leading them to the sofa. Cassie might act like she knew where she was going, but she had never visited his apartment.</p><p>Acting like she knew everything was one of the many charming traits Cassie shared with Regulus.</p><p>Which. Speaking of Regulus.</p><p>“What’s wrong with him? What happened?” When that made Cassie clam up tighter, he tried a different tack. “How can I help?”</p><p>Cassie shuddered at the question, seemingly unmoored by it.</p><p>“Siri,” she rasped through a dry throat. “Can you call Lily?”</p><p>Could <em>he</em>, Sirius Black, best friend of James Potter – fella who had long been in love with Lily Evans, call Lily?</p><p>Yes, he could. Would she answer? Well that was the question, wasn’t it?</p><p>But this was urgent. Sirius felt the little humor he’d tried to muster drain away at the face of Regulus’s steadily weakening pulse. Cassie might have acted calm, but he also knew that look. That was the look of a person who didn’t know what to do anymore.</p><p>“Yes, I can,” he said, hiding away his uncertainty. “And you can rest now, Cassie. I’ll take care of everything.”</p><p>Just like that, the manic, feverish light in Cassie’s eyes went out and she slumped sideways on the sofa. Sirius couldn’t even take a second to feel touched. Regulus went a touch paler and he knew he was almost out of time.</p><p>“James!” he shouted, hands going to his pocket. “James! Answer, dammit!”</p><p>Seeing his best friends’ bewildered face in his mirror was a minor relief. He could handle anything as long as James was with him.</p><p>“Can you get Evans? And maybe Remus? Please. I don’t have a second,” he asked.</p><p>James didn’t ask, or argue. He just vanished from the mirror and Sirius knew that was taken care of.</p><p>Sirius converted the sofa into two beds and vanished the twins’s clothing. And he almost vomited. Crusted on Cassie’s leg and Regulus’s shoulder was a bite. It looked terrible and was radiating black venom into their bodies.</p><p>That same black venom was also in Regulus’s stomach.</p><p>With a silent thanks to the Prewett Twins for teaching him the spell, he made Regulus vomit, emptying his stomach. He gagged at the rancid smell and he capped a sample and banished it.</p><p>Then they started to shiver, so he conjured a blanket and covered them. He couldn’t think to put them in clothes yet, the bite had to be treated and he had no idea how the hell to even treat it.</p><p>Just as he was starting to pull at his hair, the door opened and <em>five</em> people spilled through.</p><p>He was only expecting three.</p><p>Sirius didn’t even know when he had his wand up, but he had it up and pointed to the intruders before he could think harder.</p><p>“James?” he asked, voicing a warning.</p><p>“Padfoot!” James cried.</p><p>Which. Was almost as good as a confirmation. He relaxed marginally. “Who’s that with you?”</p><p>The other extra two people turned out to be Mr. and Mrs. Potter. The sight of actual functioning responsible adults almost made him sag in relief. Almost, because Cassie trusted him with Regulus enough to pass out in his sofa and he was going to requite that trust.</p><p>Evans and Remus flew to the twins with a gasp, but their faces were sure and their wands didn’t shake. It still didn’t sit right with him not to bring them immediately to Saint Mungos. But if the twins could be treated in Saint Mungos, Cassie would have brought them there. Instead, she’d chosen to go to his apartment, miles and miles away from London and whatever trouble that the twins had found. That meant that they’d found <em>something</em> incriminating. And they had staked their lives on it.</p><p>“This is the Draught of the Death Creeper!” Lily gasped at one point, making everyone look at her with dismay. “It’s incredibly rare and no one is supposed to brew this.”</p><p>“Some of those ingredients are also incredibly illegal,” Mr. Potter opined, eyes narrowed behind his glasses.</p><p>If there was anyone who’s opinion he trusted about potions, it would be Mr. Potter.</p><p>“Is there a cure?” Sirius asked.</p><p>“It was a poison brewed with the thought of making it impossible to have an antidote,” Mr. Potter said, standing up and starting to make a list. “Here, look.”</p><p>Sirius went a little green at the ingredient list. Beside him, James made a rude sound. Some of those ingredients had antidotes that were completely <em>incompatible</em> when mixed together. No wonder making an antidote was verging on the impossible.</p><p>Before despair and grief could hit Sirius, Mr. Potter continued, “Fortunately, we know someone who has Pheonix tears.”</p><p>“You want to owe him a favor?” Mrs. Potter asked sharply, wand making complex figure of eights over Regulus’s shoulder.</p><p>“He <em>owes</em> me a favor,” Mr. Potter corrected. “I’m just collecting.”</p><p>Mrs. Potter relaxed and Sirius hadn’t even noticed she wasn’t calm. James’s mum was really incredibly scary.</p><p>With a solution on hand and Cassie actually regaining some color in her cheeks, Sirius sat down and passed out.</p><p>.</p><hr/><p>.</p><p>Sirius was a bit too young to remember, but he did know that when the twins were born, Cassie wasn’t expected at all.</p><p>They’d gone to the medi-witch and known that they were expecting a boy. They didn’t notice Cassie hiding behind Regulus until she was coming out, to everyone’s shock, including Mum’s.</p><p>In his surprise, Orion Black had named her after the first female name he could think of, and it was Cassiopeia. That Great Aunt Cassiopeia, the Eldritch Horror of the Black Family, was still alive to be flattered and honored was just a complete coincidence.</p><p>The thing was, nobody had expected Cassie at all and so no one prepared for her.</p><p>Regulus, they had planned for. They had his room and they had his clothes and potions. They even had his toys pre-planned.</p><p>For Cassie, they had nothing. Even if they were wizards that could transfigure things with a flick of a wand, it was still something of a shock. As such, for the first three days of her life, she shared the same bed as Regulus and wore the same clothes. After three days of intense renovation of the Black penthouse, Cassie was moved to her new room and there she threw a tantrum.</p><p>The absence of her constant companion threw her for a loop and she was very confused.</p><p>Sirius still posits that those three days of cradle sharing did something, because Cassie, while growing up, was rabidly overprotective of Regulus, to the point where Regulus couldn’t even be bothered about it anymore.</p><p>Yes, the sky was blue, there was magic and Cassie would kill anything that would hurt Regulus.</p><p>These were the facts of life, and then Hogwarts happened and the twins were sorted into different houses.</p><p>.</p><hr/><p>.</p><p>A day after Cassie knocked on his door and with seven people camping in his living room, Regulus woke up first.</p><p>Sirius was still baffling about this, even as he hugged Regulus tightly. He was dazed and disoriented but he hugged back, conditioned to hugs by Cassie for years.</p><p>“You’re awake, Morgana’s fucking tits, you’re awake,” he said to Regulus’s shoulder.</p><p>Regulus awkwardly patted him on the back.</p><p>“I can’t believe you woke up first,” Sirius said into his shoulder. He was doing his damnest not to cry because Regulus had opinions about <em>leaking</em>. “How do you feel?”</p><p>Regulus opened his mouth to answer and coughed. Sirius scrambled out of the hug to find him a glass of water.</p><p>“Here, drink slowly.”</p><p>He did not drink slowly. He drank fast, like it was the only thing he needed to live. Like he hadn’t drunk water in years. He coughed, but he determinedly continued.</p><p>Sirius fluttered his hands, unsure of where to touch. The diagnosis that Lily had left made him feel a bit faint, but he supposed that phoenix tears would solve that.</p><p>“No really,” he said sternly. “Your stomach lining and esophagus suffered from potion burns and was almost infected. Drink slowly.”</p><p>Regulus slowed down.</p><p>“How’s Cassie?” he rasped.</p><p>Typical. He didn’t know what he should have expected. On the verge of death and still worrying about each other.</p><p>“Cassie is fine. In better shape than you,” Sirius said, channeling Remus at his most serious mood. “She only had one fractured rib, an infected necromantic bite and magical exhaustion. Mrs. Potter fixed up that bite and Remus took care of the rib. She’ll have to sleep off the magical exhaustion. You on the other hand…”</p><p>Regulus’s case was terrible and Sirius was forever in Mr. Potter’s debt for using a favor to get phoenix tears.</p><p>“You almost died, Reggie,” Sirius whispered. “You had poison in your blood stream and in your stomach. Your digestive tract was almost shredded and infected. That necromantic bite was interacting with the poison. <em>You almost became an Inferi in my sofa</em>.”</p><p>Regulus held Sirius’s hand. He didn’t know for who’s comfort, because just thinking about it, Sirius wanted to scream.</p><p>“You were inches away from it. If Evans hadn’t noticed the chemical mixture – “</p><p>“Sirius,” Regulus interrupted what was about to be hysterical ranting. “Thank you.”</p><p>Sirius’s breath hitched and then he broke down and cried, face down on his baby brother’s lap. When James woke up, that’s how he found them.</p><p>.</p><hr/><p>.</p><p>If Sirius getting to Gryffindor was a shock it was nothing compared to the complete and utter dismay that Cassie had when the Hat shouted Ravenclaw for her and Regulus got sorted into Slytherin.</p><p>He’d been half afraid she would throw a tantrum but though her cheeks flushed with outrage and her silver eyes flashed, she didn’t say a word. She watched Regulus, who just shrugged and smiled at her with resignation.</p><p>Thinking back on it, maybe he should have expected trouble when she lost her anger and instead started looking <em>thoughtful.</em></p><p>Regulus did look a bit panicked at the look on Cassies face, which had been funny. Knowing what Cassie did when pushed into a corner, he might have panicked along with Regulus. Funny how hindsight and regret always came after the first explosion.</p><p>After three days, Cassie knew the Slytherin Common Room password. Sirius had been trying for a year and she does it in <em>three days.</em></p><p>When that changed after she infiltrated it, Cassie broke down the Common Room wards and let herself in anyway.</p><p>Sirius heard all about it because sometimes, the twins like to think he’s a neutral party and go to him to complain about each other.</p><p>“Sirius,” Regulus whined. “I love her but she just placed something in my clothes that made the hexes others shoot at me to bounce back at them.”</p><p>“That’s great, why didn’t I think of that?” Sirius muttered without thinking.</p><p>Regulus shot him a look of disgust that also seemed to be fond at the same time.</p><p>“Sirius,” Cassie whined at him a few days later. “Regulus won’t hug me anymore.”</p><p>He blinked at her and her bright red hair. “Why are you bright red?”</p><p>“I went to hug him in the Slytherin table for my breakfast Regulus hug and he hexed me.”</p><p>Sirius had to bite the inside of his cheek so he wouldn’t laugh.</p><p>“He was probably embarrassed,” Sirius said when he could speak without giggling. “Try hugging him when he’s in private.”</p><p>Sirius just had to prank somebody, because his siblings were ridiculous. James Potter was a gift. He listened without judgement and didn’t even blink when Sirius sometimes muttered about the adorable little menaces he had as siblings.</p><p>.</p><hr/><p>.</p><p>Cassie woke up the next day and unfortunately, the first sight she had was Regulus vomiting black blood and torn stomach tissue into a bucket.</p><p>Part of the healing process, Mrs. Potter had announced but it didn’t make it less disgusting. Also terrifyingly adorable, because Cassie was dazed from healing but still ready to hex anything in under a second.</p><p>James shot him an exasperated look, likely guessing what he was thinking about and Sirius pulled himself together. Prevent murder now, coo over her adorable murderous tendencies later.</p><p>“Cassie, he’s fine,” Sirius soothed. “He’s perfectly healed, but all those things don’t go away somewhere so he has to vomit them back up.”</p><p>Cassie looked at Regulus, then at Sirius and then everyone else. She lowered her wand shakily, adrenaline gone and sat down at the edge of her bed weakly. “He’s really okay?” she asked slowly, quietly.</p><p>“Yup,” Sirius said as cheerfully as he could manage. “Not a thing to be worried about.”</p><p>Cassie’s hands worked on her blanket and her eyes never left Regulus’s vomiting form. “His pulse stopped on me while I was still figuring out where to go,” she said. Her eyes were slightly glazed and he did not like the look of that. “I had to restart it with a spell.”</p><p>“Phoenix tears work magic,” Sirius said. “Cassie, here. Have some water.”</p><p>Cassie had always been the bolder one of the twins, the one who doled out hugs and kisses but also handed out retribution without a thought. A fiery volcano that, once exploded, would likely forget about the offense.</p><p>That Cassie was subdued and tears slowly rolled down her cheeks as she drank her water. The image was just <em>wrong.</em></p><p>“What happened, Cassie?” he asked her gently. </p><p>Conveniently, Regulus stopped vomiting and gagged at the taste in his mouth. He accepted the cup of water he was handed gratefully and cleaned his mouth of the taste.</p><p>“There was a lake of Inferi,” Regulus volunteered. His voice had a raspy quality it had never had before. Mr. Potter said that it would likely have that rasp forever.</p><p>“Why did you go to a lake full of Inferi?” Mrs. Potter asked quietly.</p><p>“Because You-Know-Who had something important hidden in the center,” Cassie answered. The glassy look was gone from her eyes, but she looked terrified instead, which was worse. “A locket.”</p><p>“A horcrux,” Regulus added. “Under a potion.”</p><p>“And you had to drink it because it couldn’t be vanished,” Cassie said, then she burst into tears.</p><p>Half a dozen hands were ready to touch her, but Regulus got there first. Half-healed from such terrible poison damage that it took phoenix tears and groggy from sleep and medication, Regulus got there first. He staggered into her back and placed his face at the back of her neck. His hands held hers tightly and both of them swayed. Cassie cried, tears reduced to quiet sobbing with Regulus's hug.</p><p>“I’m alright,” he could hear Regulus say to her. “We’re alright. We got out. We’re alive.”</p><p>"We're alive," Cassie whispered back through hiccups.</p><p>.</p><hr/><p>.</p><p>The twins loved each other very much and it was obvious to anyone who looked.</p><p>They didn’t have any arguments and when their opinions differed, they respected that moved on. Evans had once commented that it was healthier than any relationship she’d ever seen.</p><p>“It’s a relationship of equals,” Evans explained to him and the boys why she liked the twins even if other people thought different. “You only read about a partnership like that once in your lifetime.”</p><p>Sirius didn’t really think much about it until they had their first serious fight in the twins's fifth year.</p><p>There wasn’t any shouting or screaming or even hexing.</p><p>Cassie simply didn’t go out of her way to find Regulus and when they did meet each other, was frostily polite to the point of rigidity.</p><p>Regulus shot her longing looks but didn’t say anything, simply bowing his head and accepting her cold temper.</p><p>Everyone watched this with bated breath. The Black Twins, arguing?</p><p>Some factions tried to shoe-horn into their relationship while it was fractured, but the twins were still rabidly overprotective of each other even while angry at the other. Nott learned that the hard way after Cassie blasted him down three flights of stairs for daring to recruit Regulus for You-Know-Who. And several boys were hexed to being unrecognizable after asking her out on a date by a very angry Regulus.</p><p>After a month of this coldness, the twins resolved their one and only argument and went back like nothing happened.</p><p>Sirius, of course, knows nothing about what they argued about. Only that Cassie was angry at Regulus and that Regulus had, for once, forbidden her from doing something.</p><p>In hindsight, he should have asked.</p><p>He really, truly should have asked.</p><p>.</p><hr/><p>.</p><p>Sirius was for once glad that there were other people in his flat to hear this. He knows he’s a private person, but fucking Merlin, other people needed to hear about the twins’s stupidity.</p><p>“We started researching about the You-know-who when were in first year,” Cassie explained candidly, hands still entwined with her twin and not moving any time soon. “Regulus stumbled on horcruxes at one point and we realized that he was <em>exactly</em> the sort of person who would have one, if not more. Then, we discovered his cave of Inferi’s and investigated it in the Summer of our Fifth Year. When we realized what we had to do to <em>get</em> it…”</p><p>“We flipped a coin,” Regulus said.</p><p>“You flipped a coin,” Lily repeated, aghast.</p><p>“So that’s why you had an argument in your Fifth Year?” James asked.</p><p>Regulus looked sheepish while Cassie glared daggers at him. “Yes,” Cassie said. “I begged him to be the one to do it, but he said that if I wanted to protect him, it was making sure that he was alive afterwards.”</p><p>Sirius winced. Yikes. So that’s why Cassie was so angry at Regulus at that time.</p><p>“Why didn’t you tell anyone?” Mr. Potter asked.</p><p>The twins looked surprised at the question and Sirius leaned forward, interested in hearing the answer.</p><p>“I suppose,” Regulus said while Cassie looked down. “That we got used to no one looking after us but each other. We had Sirius but he had his own problems. We only had each other.”</p><p>While touching, Sirius wanted to pull his hair out.</p><p>Merlin’s fucking saggy – !</p><p>“I’m not going to scold you,” Mr. Potter said in that calm even tone that had James reflexively hunching his back in fear of remembered lectures. “Because this is clearly a product of you growing up without proper adults to look after you. But we’re right here now. So don’t do this again.”</p><p>“Sirius had the same problem,” Mrs. Potter added, making Sirius flinch down with James. “He once tried to hide a broken bone because he didn’t want to bother anyone.”</p><p>“It won’t be a bother to us,” Mr. Potter said and meaning it very clearly. He looked them in the eyes sincerely. “I want to know. I’m interested. I won’t turn you away if you want to talk to me, even if I’m busy. I'll make time for you.”</p><p>The twins stared at Mr. and Mrs. Potter with wide eyes filled with awe over sentiments that should have been <em>normal.</em> He had been like that once, before repeated visits to the Potter Manor made him realize that his parents were the ones who were different.</p><p>Looking at the two of them and wondering what else he failed them in, Sirius <em>ached.</em></p><p>“It’s not your fault,” James whispered to him while they all watched the twins get more or less adopted by James's parents.</p><p>It wasn’t Sirius’s fault, but he will do better. Looking at everyone around them, he knows the resolve is not just his own. They will do better. They have to.</p>
  </div><div class="fff_chapter_notes fff_foot_notes"><b>Author's Note:</b><blockquote class="userstuff"><p>I love Cassie. She's like me, with no impulse control.</p><p>Comments please!</p><p>Edited on 4/7/2021: because I tend to re-read my works and cringe. Ah, the problem of not doing re-reads of what you write before posting it...</p></blockquote></div></div>
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